1. Field of the Invention
The present invention relates to active inrush current circuits and, more particularly, to active inrush current circuits in power factor correction control circuitry.
2. Prior Art
Power supplies connected to an AC line voltage supply, are often subjected to short-duration, high amplitude, input current (known as inrush current). The inrush current may be many times the steady state current until the power supply reaches equilibrium, i.e., the transient effect continues until the voltage across the internal power supply capacitance reaches a voltage approximately equal to the peak amplitude of the AC line voltage supply. If uncontrolled, the inrush current can result in internal power supply capacitance absorbing energy beyond its rated value as well as subjecting power supply components to damaging current levels.
One approach to controlling inrush current uses a relay as the current control device. FIG. 1 shows one implementation of inrush current control using a relay. At first application of the AC mains voltage, the current to charge the output capacitor Cout is limited through the resistor, RLIM. The AC line is rectified by diode bridge DB1. The decision to close the relay originates in inrush control block K2. The control function could be that the voltage impressed on Cout has stopped rising.
the inrush control block K2 could also monitor the peak value of the input voltage and compare that to the output voltage on Cout and close the relay when the difference is within a predetermined range. When this occurs, inrush current has decayed to normal operating levels and the relay K1 can be closed by a control signal from the UPFC IC with minimal or no surge current through the relay K1. At this point, down stream converters, whether PFC Boost converters or just load DC/DC Converters can be enabled. In the case of the PFC, the control circuit will limit the input current used to charge a bulk capacitor above the input voltage to the regulation point. Although not shown here, many times a second comparator is used to detect that the output voltage is charged to a predetermined minimum level, i.e. the peak AC input at low line; this signal may be logically combined with the inrush control block K2 to control the relay K1 turn-on. However, it is readily apparent that during shutdown, or line dropouts the relay will not open until the output voltage is lower than the input voltage or the absolute value of the output voltage is below a preset minimum. In addition, it is often desired that the circuit detect cycle drops and maintain operation. However, if the applied AC is at high-line, a cycle drop-out could lead to high surge currents, especially if the Boost power supply is still running. The only current limiting functions would be the series resistance of the circuit through the output capacitor Cout. For example, if with a bulk voltage Vbulk of 150VDC, a peak line voltage of 350VAC, and a series resistance of 1 ohm, the surge current could easily reach hundreds of amperes destroying many of the power supply components. It is also readily appreciated that many relays have pull-in/drop-out times measured in milli-seconds. This slow reaction time can cause the same problems as discussed above.
Another method for controlling inrush current is to use a series connected negative temperature coefficient (NTC) resistive device. The NTC will limit inrush at start-up, but once it's hot, the resistance is low until the device has had time to cool. Thus, in situations where power cycles on and off before the device has had time to cool, there will be minimal, if any, inrush current protection.
Another circuit for controlling inrush current may use a silicon controlled rectifier (SCR). Referring to FIG. 2, a circuit schematic implementing an SCR based inrush current control scheme is shown. This circuit uses an SCR bridge X1,X2 with separate diodes and resistors for the inrush path. Inrush is sensed as in the circuit of FIG. 1 and the gates of the SCRs X1,X2 are modulated on or off by the inrush control block 20. It is readily appreciated that the SCRs can transition from full on to full off and back again in microseconds, as opposed to the slower milliseconds response of the relay approach to controlling inrush current. The disadvantage, however, is that two otherwise low forward drop diodes are replaced with two high forward voltage drop SCRs. These components are in the path of the input current, which is the highest average current path in the boost converter, resulting in an adverse effect on efficiency. In addition, drive for the SCRs are another disadvantage. The most conventional approach drives the SCRs with a current source equivalent. The gate loss in this case can be high. Moreover, the benefit to placing the SCR in the input current path is that the current is essentially continuous and drive is only needed around the zero crossing. This will be true for all conditions that keep the minimum holding current of the SCR flowing. But, the holding capability of the SCR can cause delay in the turn-off response time of the circuit.
Power metal oxide silicon field effect transistor (MOSFET) are also often used as current control switches. However, power MOSFETs have many undesirable features such as conduction characteristics that are strongly dependent on temperature and voltage ratings. Moreover, MOSFETs are largely insensitive to MOSFET gate voltages, and for large values of drain current, the drain to source voltage is primarily a linear function of the drain current. This resistive effect limits the usefulness of MOSFETs to low current applications.